Electrolytic production of ductile chromium



United States Patent ELECTROLYTIC PRODUCTION OF DUCTILE .CHROMIUM Eugene Wainer, Cleveland Heights, Ohio, assignor, by mesne assignments, to Horizons Titanium Corporation, Princeton, N. J., a corporation of New Jersey No Drawing. ApplicationOctober 17,1955 Serial No. 541,067

3 .Claims. Cl. 204-64 This invention relates to the art of producing ductile chromium. More particularly, it relates to the preparation of ductile chromium by a process in which a double fluoride of chromium and an alkali metal is electrolyzed in a fused salt electrolyte comprised of at least one alkali metal halide.

For purposes of illustration the process will be described for the electrolyses of K CrF in a melt of NaCl, but it -will be understood that other compounds which produce the same result may be substituted for the K CrF or NaCl Without departing from the intended scope of this invention.

Chromium resembles many metals in the fact that the properties of elemental chromium are radically altered by the presence of small but significant amounts of impurities. Heretofore there have been developed a number of elaborate techniques directed to the production of relatively pure chromium, characterized by ductility. Known methods include decomposition of the tetraiodide, deposition from aqueous electrolytes, and reduction of various halides by suitable metals: The product of these methods is generally subsequently purified by a further treatment with hydrogen at elevated temperatures.

One objection to these prior art methods has been the necessity for precise control of the process variables and for the subsequent purification to produce a product with the desired ductility.

One of the objects of the present invention is to provide a simple and efficient method of producing ductile chromium.

Another object of this invention is to provide a simple and direct electrolytic method of producing ductile chromium.

Another object of this invention is to provide a method of producing chromium metal in the form of crystalline dendrites which may be readily consolidated and thereafter worked into sheet, rods, wire and other desired shapes.

These and other objects will be apparent from the following description.

Among the procedures suggested in the prior art for the winning of metallic chromium from its compounds, are the fused salt electrolyses patented by Driggs and Marden in U. S. Patent 1,821,176. This patent discloses a procedure in which a fused bath based on an alkaline earth metal halide, such as calcium chloride, to which a suitable source of chromium has been added, is decomposed by passage of. an electric current through the bath. The metal is ultimately recovered by suitable washing techniques. I have found that the alfinity for water possessed by alkaline earth metal halides is so great that it is Virtually impossible .toobtain an oxygen-free anhydrous bath based on a member of this class of compounds. Furthermore, the separation of alkaline earth metal compounds from metallicchromium is rendered tficu t by h w solubi i y posse sed by such compounds, in comparison with the relatively greater solubility of most alkalimetal halides. Whatever the reasons,

it has been found that to obtain ductile chromium, it is necessary for the product to be further purified, to remove oxygen or other contaminants which renderit brittle. Another prior art approach is disclosed in U. S. Patent 1,861,625 in which the process described :involves the electrolysis of a fu sed salt bath, preferably one containing aa alkaline earth halide, under the protection ;.of .an alkali metal fog produced by the electrolytic decomposition of one or more constituents of the fused salt bath.

In accordance with this invention .a fused salt bath is prepared, free :from moisture or other oxygen containing substances, by melting tone or more alkali metal chlorides in a controlled environment. ,Preferably, the fused melt is formed of :sodium chloride since this compound is commercially available with the desired purity and at low cost. Mixtures of sodium chloride and potassium chloride, or of other halides may also be employed, but as these merely add to the expense of the process without any attendant advantages, sodium chloride ordinarily is preferred as the sole constituent of the fused melt.

The fused melt serves as a carrier for a source of chromium. Because it is readily obtained in relatively :pure anhydrous and oxygen-free form and because it is stable at elevated temperatures, the compound K Cr-F is preferred as theionizable source of chromium in the electrolytic process by which ductile chromium is obtained as a cathode deposit. This salt melts at temperatures between 750 C. andSSO C. With little: decomposition. It is also quite stable in :air and does not hydrolyze or oxidize appreciably when stored at room temperature.

The electrolytic cell employed in the instant process is similar to that described in the Journal of the Eelectrochemical Society, vol. 101, page 63 et seq., and vol. 102, pageZSZ et seq., for the production of other metals. It is characterized by .an all graphite interior in which the fused salt melt is confined. The graphite crucible serves :as the cell anode and is usually contained ina lampblack insulated graphite cylinder supported by a steel shell. The cell has a cover provided with openings for the inlet and outletof an inert atmosphere, such as argon gas or helium gas and for the insertion and for Withdrawal of one or more cathodes from the melt. Suitable electrical means are provided for impressing a potential between the anode and one or more cathodes in order to conduct the desired electrolysis. Steel has been found to be a suitable metal for the cathode, although other metals including molybdenum, tungsten, iron or nickel may be used in place thereof.

In actual operation, the cell is first thoroughly evacuated under 'heat to insure the removal of all moisture and volatile contaminants from the graphite parts and from the lampblack surrounding the crucible. Following evacnation, the cell is flushed with argon or other inert gas. After the cell has been suitably prepared, sodium chloride is charged into the cell and melted therein. The term perature is maintained at about 800 C. under the inert atmosphere. An ionizable chromium compound, for example, K CrF is then added to the melt and after it has been dissolved therein as evidenced by the formation of a lump-free clear melt, a steel cathode supported by a graphite-sheathed nickel rod is lowered into the melt under a potential of 2 to 3 volts.

The melt is next purified by electrolytic and gettering action. This is accomplished by means of a short we electrolysis under normal operating conditions, subsequently described. Between 2 and 12 ampere hours per pound of melt is generally sufiicient to accomplish the desired removal of impurities and after such a time, the cathode. is removed and a fresh cathode inserted in its place. More extensive tire-electrolysis is not ordiasagose 3 narily required unless the contaminated.

The deposition of ductile chromium in the form of crystalline dendrites is effected under an insert atmosphere at between about 3 and about 5.5 volts and 180 to 500 amperes per square decimeter initial cathode current density by decomposition of the K CrF in the fused salt melt. Under the actual operating conditions in the presence of substantial amounts of chromium, these values are Well below those at which any appreciable decomposition of alkali metal bath constituents would occur with the formation of any protective fog at the oath ode. The melt is maintained at a temperature between bath is exceptionally 750 and 850 C. for optimum electrolytic deposition. Con- I centrations of between and weight percent of K CrF have been found suitable. The substantially complete removal of chromium from the melt is signaled by a characteristic voltage increase which may be accompanied by the production of free alkali metal at the cathode. melt and allowed to cool in the inert atmosphere blanketing the cell.

After it has cooled sufficiently, the deposit is removed from the cell and is chipped olf the cathode, crushed lightly and washed to separate the chromium metal dendrites from the alkali metal salt constituents of the fused bath. The deposit is first broken up into lumps between A; to 6 inch in size and leached in warm Water until fluoride and chloride ions are no longer detected in the wash water. Subsequently, the deposit is further treated to remove additional impurities, which are principally hydrolytic products formed during the electrolysis or subsequent washing operation, from any residual chromium salts. Various inorganic acids have been employed including concentrated hydrochloric acid which etched the metal slightly and nitric acid which tended to produce an oxide film on the metal. Mixtures of concentrated nitric acid and hydrofluoric acid appear to be most suitable for washing the deposit. As recovered, fused salt electrolytic chromium crystals are dendritic and cubic in nature and grow up to /1 inch in length. They are single crystals in general. Individual crystals of the elec- At thispoint the deposit is withdrawn from the trolytic material may be bent cold up to 120 without tration and are not to be considered limitative of the invention.

EXAMPLE I The cell was evacuated and then flushed with argon as above described. Sodium chloride was added to the cell and heated to about 800 C. to form a molten pool. Suflicient K CrF was added to the molten NaCl to provide a concentration of about 25% by weight of K CrF After a clear melt had formed, a steel cathode was lowered into the bath under a potential of 2.5 volts. The bath was purified by a pro-electrolysis conducted at 500 amperes/squarc decimeter cathode current density and a cell voltage of 5 volts until about 12 ampere hours/pound of melt had been passed through the bath. The cathode was withdrawn and a fresh cathode was inserted in its place. Electrolysis was continued at an initial current density of about 500 amperes/ square decimeter based on the initial cathode'area, The voltage varied between 3.6 and 5.4 volts during the run as deposit built up on the cathode. The bath was maintained under an'inert argon atmosphere, and at a temperature ofabout 830" C.

. 4 After suflicient chromium had been deposited, the electrolysis was terminated and the cathode was withdrawn. Crystalline dendrites of chromium were separated from the salts in which they were embedded by the wet separation above described. The product was found to have a hardness of Rockwell B and a yield of 69% was obtained at a current efliciency of 19.5%.

EXAMPLE II The cell was prepared as in Example I and the bathof Example I was remelted in the same cell with additional NaCl and K Cri to produce a melt with an initial concentration of K CrF of about 20% by Weight. A preelectrolysis for about 7 minutes at amperes, to provide about 13 ampere hours per pound of bath preceded the main electrolytic run. After the impurities had been removed on the original cathode and a fresh cathode had been inserted, the electrolysis was begun at 5.2 volts and 420 amperes/square decimeter of initial cathode area. Polarizations which occurred during the electrolysis necessitated a lowering of the current density to about amperes /square decimeter of cathode area with an accompanying drop in the voltage to 3.6 volts. During this run the bath was maintained at a temperature of about 750 C. Upon completion of the run, the product was recovered as in the previous example and after being washed free of adhering salts, it was consolidated by are melting to produce a button having a hardness of 60 (Rockwell B). In this mm the yield was 96% and the current efficiency was 25%.

The product metal is comparable to the best chromium prepared by other methods. Analysis of the metal obtained in Example II was as follows:

Electrolytic chromium Impurity Percent Si 0.005 Fe 0.008 Mg 0.001 Mn 0.002 A1 0.002 V 0.05 Sn 0.001 Cu 0.0005 Ti 0.001 Ni 0.001 Ca 0.005 Pb 0.002 0 0.12 H a- 0.001 N: 0.001 C 0.015 S 0.002

Not detected by spectrographic analysis: Cd, B, P, Sb,

As, Mo, Zn, Zr, Co.

Lclaim:

l. Themethod of preparing ductile chromium which comprises forming a fused salt bath consisting essentially of at least one alkali metal chloride, adding an ionizable double fluoride of an alkali metal and chromium to said fused bath in an amount between about 15% by weight and 35% by weight, maintaining the bath at a temperature between about 750 C. and 850 (3., providing an inert atmosphere over the resulting melt, passing a direct current between an anode and a cathode to remove impurities including oxygen from the melt, replacing the cathode with a fresh cathode and maintaining a voltage and cathode current density below that at which free alkali metal forms at the cathode but above the minimum necessary to effect the decomposition of the chromium compound in the bath, thereby depositing chromium in the form of ductile dendritic crystals on the cathode, and recovering the crystals of chromium from the cathode deposit. 7 p

2. The method of preparing ductile chromium which comprises forming a fused salt bath consisting essentially of at least one alkali metal chloride, adding an ionizable double fluoride of chromium and an alkali metal to said fused bath in an amount between about 15% by weight and 35% by weight, maintaining the bath at a temperature between about 750 C. and 850 0, providing an inert atmosphere over the resulting melt, passing a direct current between an anode and a cathode to remove impurities including oxygen from the melt, replacing the cathode with a fresh cathode and maintaining a voltage between about 3 and about 5.5 volts and a cathode current density between about 180 and about 500 amperes per square decimeter, which voltage and current density are below those at which free alkali metal forms at the cathode when chromium is present in the bath but above the minimum necessary to effect the decomposition of the chromium compound in the bath, thereby depositing chromium in the form of ductile dendritic crystals on the cathode, and recovering the crystals of chromium from the cathode deposit.

3. The method of preparing ductile chromium which comprises forming a fused salt bath consisting essentially of sodium chloride, adding a double fluoride of chromium, K crF to said fused bath in an amount between about 15% by weight and 35% by weight, maintaining the bath at a temperature between about 750 C. and 850 0, providing an inert atmosphere over the resulting melt, passing a direct current between an anode and a cathode to remove impurities including oxygen from the melt, replacing the cathode with a fresh cathode and maintaining a voltage and cathode current density below that at which free alkali metal forms at the cathode but above the minimum necessary to effect the decomposition of the chromium compound in the bath, thereby depositing chromium in the form of dendritic crystals on the cathode, and recovering the crystals of chromium from the cathode deposit.

References Cited in the file of this patent UNITED STATES PATENTS 1,842,254 Driggs Jan. 19, 1932 1,861,625 Driggs et a1. June 7, 1932 2,731,402 Topinka et al. Jan. 17, 1956 

1. THE METHOD OF PREPARING DUCTILE CHROMIUM WHICH COMPRISES FORMING A FUSED SALT BATH CONSISTING ESSENTIALLY OF AT LEAST ONE ALKALI METAL CHLORIDE, ADDING AN IONIZABLE DOUBLE FLUORIDE OF AN ALKALI METAL AND CHROMIUM TO SAID FUSED BATH IN AN AMOUNT BETWEEN 15% BY WEIGHT AND 35% BY WEIGHT, MAINTAINING THE BATH AT A TEMPERATURE BETWEEN 750* C. AND 850* C., PROVIDING AN INERT ATMOSPHERE OVER THE RESULTING MELT, PASSING A DIRECT CURRENT BETWEEN AN ANODE AND A CATHODE TO REMOVE IMPURITIES INCLUDING OXYGEN FROM THE MELT, REPLACING THE CATHODE WITH A FRESH CATHODE AND MAINTAINING A VOLTAGE AND CATHODE CURRENT DENSITY BELOW THAT AT WHICH FREE ALKALI METAL FORMS AT THE CATHODE BUT ABOVE THE MINIMUM NECESSARY TO EFFECT THE DECOMPOSITION OF THE CHROMIUM COMPOUND IN THE BATH, THEREBY DEPOSITING CHROMIUM IN THE FORM OF DUCTILE DENDRITIC CRYSTALS ON THE CATHODE, AND RECOVERING THE CRYSTALS OF CHROMIUM FROM THE CATHODE DEPOSIT. 